Alumina ceramics are one of the most widely used technical ceramics. Alumina ceramics show excellent hardness which means, in turn, excellent wear-resistance. They are used in applications such as mechanical seals, yarn guides and the like. Alumina ceramics show, however, severe brittleness, which often necessitates toughening of the ceramics. The transformation toughening by ZrO.sub.2 in an alumina matrix is quite well known. Rather recently, SiC whiskers have also been widely used to toughen alumina ceramics. However, several problems have been observed in the following areas;
i) Difficulty in obtaining homogeneous mixing of SiC whiskers with alumina powder and difficulty in full densification of the mixture; PA0 ii) Instability of SiC whiskers in oxidizing environments; and PA0 iii) Health hazards presented by SiC whiskers.
.beta.-,.beta."- and .beta."'-aluminas are well-known and have been discussed frequently in the literature. For example, see (1) Bettman, M. and Terner, L. L. "On the structure of Na.sub.2 O.4MgO.15Al.sub.2 O.sub.3, a variant of .beta.-alumina", Inorg. Chem. 10, 1442, (1971), (2) Kummer, J. T. ".beta.-alumina electrolytes" (Progress in Solid State Chemistry) Ed. Reiss, H. and McCaldin, J. O. PP. 141-75, (1972), (3) Reijnen, P, and Kim, H. D. "The influence of Na.sub.2 O on sintering and creep of alumina doped with MgO and TiO.sub.2 " cfi/Ber. DKG 63, 272-79, (1986), (4) Reijnen, P. and Kim, H. D. "Microstructure development in alumina" VIII. Yugoslav-German Meating on Materials Science and Development, May, (1987) pp. 91-97 and (5) Kim, H. D. and Reijnen, P., "Effect of Na.sub.2 O on alumina sintering" The 10th Korea Symposium on Science Section, pp. 179-95.
When sodium and magnesium (Na/Mg=1/2) are present as impurities in alumina sintering, sodium aluminate (.beta.-alumina, NaAl.sub.11 O.sub.17) and magnesium spinel (MgAl.sub.2 O.sub.4) are formed at relatively low temperatures such as 1100.degree. C. and, as the temperature increases, .beta.-alumina and Mg spinel react with each other to give .beta."'-alumina by the following reaction; EQU NaAl.sub.11 O.sub.17 +2MgAl.sub.2 O.sub.4 =NaMg.sub.2 Al.sub.15 O.sub.25
(.beta.-alumina) (Mg spinel) (.beta."'alumina)